1. Field of the Invention
This invention relates to a steel wire rod for cold forging used for manufacturing machine structural components such as car and construction machine components and the like, and a method to produce the same.
2. Description of the Related Art
Machine structural components such as those of cars and construction machines, for example bolts, stabilizers, and the like, have been manufactured conventionally through the cold forging of steel wire rods of carbon steels for machine structural use or alloy steels.
In other words, steel wire rods of carbon steels for machine structural use or alloy steels hot rolled and left to cool to room temperature are commercially available, and secondary manufacturers manufacture the formed machine components from the steel wire rods, in most cases, through the following sequential processes: a preliminary drawing at an area reduction of 15 to 30% and then a spheroidizing annealing to secure cold workability; a finish drawing to obtain accurate dimensions and smooth surfaces; a cold forging (such as thread rolling and the like) for forming; and a heat treatment for quenching and tempering.
The reason why the preliminary drawing is conducted is that, since the structure of a hot-rolled wire rod consists of ferrite and lamellar pearlite and it is difficult to apply spheroidizing annealing to the as hot-rolled material, it is necessary to fragment pearlite lamellae (cementite) and give drawing strain to the material through the preliminary drawing as a pretreatment to the spheroidizing annealing.
However, the preliminary drawing at secondary manufacturers is not desirable, for it constitutes an additional process and thus not only lowers productivity but also increases manufacturing costs.
Various technologies to accelerate spheroidizing have been known, such as those proposed in Japanese Unexamined Patent Publications No. S56-119727, No. S57-45929, No. S60-152627, No. H6-336620, etc.
However, the technology disclosed in the Japanese Unexamined Patent Publication No. S56-119727 requires cold drawing, and another disclosed in Japanese Unexamined Patent Publication No. S59-5702 requires a rapid cooling after hot rolling to impose plastic strain on the material. Japanese Unexamined Patent Publications No. S58-164731 and No. H6-336620 disclose technologies to form a martensite structure through rapid cooling after hot rolling. However, no technologies have been disclosed in relation to applying the spheroidizing annealing without preliminary drawing and realizing high ductility as a result of such a spheroidizing annealing.
In view of the above situation, the object of the present invention is to provide a steel wire rod for cold forging which can be spheroidizing-annealed in an as hot-rolled state without preliminary drawing and rendered highly ductile through the spheroidizing annealing, and a method to produce the same.
The inventors of the present invention directed attention to the facts that, to obtain a good spheroidized structure equivalent to the structure of a wire rod subjected to the conventional process of applying preliminary drawing prior to the spheroidizing annealing, it was important to homogeneously distribute carbon in the steel structure before the spheroidizing annealing so as to reduce the distance of carbon diffusion during the spheroidizing annealing, and that a bainite or a martensite structure containing evenly distributed carbon was the most suitable for the purpose. Based on this, the present inventors discovered that it was possible to eliminate the preliminary drawing prior to the spheroidizing annealing and obtain high ductility through the spheroidizing annealing by forming fine crystal grains through a low temperature rolling in order to facilitate the carbon diffusion, and by properly designing a cooling process so that a martensite, bainite or bainite-martensite structure could be obtained; and established the present invention on the basis of the finding.
The gist of the present invention, therefore, is as follows:
(1) A steel wire rod for cold forging, characterized by: containing, in weight,
0.1 to 0.5% of C,
0.01 to 0.5% of Si, and
0.3 to 1.5% of Mn,
with the balance consisting of Fe and unavoidable impurities; and having the prior austenite grain size number defined under Japanese Industrial Standard (JIS) G 0551 being 11 or higher, the amount of diffusible hydrogen in the steel measured by the programmed temperature gas chromatography being 0.2 ppm or less, and the hardness being Hv 250 to 700.
(2) A steel wire rod for cold forging, characterized by: containing, in weight,
0.1 to 0.5% of C,
0.01 to 0.5% of Si, and
0.3 to 1.5% of Mn,
with the balance consisting of Fe and unavoidable impurities; and having the ferrite crystal grain size number defined under JIS G 0552 being 11 or higher, and the spheroidizing index defined under JIS G 3545 being No. 2 or below.
(3) A steel wire rod for cold forging according to the item (1) or (2), characterized by further containing, in weight, one or more of
0.2 to 2.0% of Cr,
0.1 to 1.0% of Mo,
0.3 to 1.5% of Ni,
1.0% or less of Cu, and
0.005% or less of B.
(4) A steel wire rod for cold forging according to any one of the items (1) to (3), characterized by further containing, in weight, one or more of
0.005 to 0.04% of Ti,
0.005 to 0.1% of Nb, and
0.03 to 0.3% of V.
(5) A method to produce a steel wire rod for cold forging characterized by hot rolling a steel having a chemical composition specified in any one of the items (1), (3) and (4) at a finish rolling temperature range from the Ar3 transformation temperature to 200xc2x0 C. above it, rapidly cooling the steel wire rod thus rolled by laying it on a conveyer in the form of continuous rings in order to form a martensite, bainite or bainite-martensite structure and, after collecting the wire rod into a bundled coil and before banding it, tempering the coiled wire rod by holding it in a furnace atmosphere controlled to a temperature range from 300 to 600xc2x0 C. for 15 min. or longer but shorter than 1 h., so that the prior austenite grain size number defined under JIS G 0551 is 11 or higher, the amount of diffusible hydrogen in the steel measured by the programmed temperature gas chromatography is 0.2 ppm or less, and the hardness thereof is Hv 250 to 700.
(6) A method to produce a steel wire rod for cold forging characterized by applying spheroidizing annealing subsequent to the production processes according to the item (5) so that the ferrite crystal grain size number defined under JIS G 0552 is 11 or higher, and the spheroidizing index defined under JIS G 3545 is No. 2 or below.